1. Field of the Invention
The present invention relates to a filtering technique carried out by using a digital filter.
2. Description of the Background Art
A weight measurement apparatus for measuring a weight of an article typically employs a weight sensor such as a strain gauge load cell or a force balance. The weight sensor has a natural frequency which is subject to a total weight of a weight of an article and a tare thereof. When external vibration at frequencies in a band including the natural frequency acts upon a weight measurement system, external vibration at a frequency close to the natural frequency is amplified, to work as a vibration noise, which then emerges in a weighing signal output from the weight sensor. It is noted that such a vibration noise will be hereinafter referred to as a “natural frequency noise” throughout the present specification.
Also, in measuring a load of a transfer system, a noise caused due to vibration of a rotation system such as a motor for driving a conveyor belt or a transfer roller, or an electrical noise associated with a commercial power supply, works as a vibration noise which is superimposed on a weighing signal.
A weight measurement apparatus developed in recent days has employed a digital filter for eliminating such a vibration noise as described above which is contained in a weighing signal. A digital filter is relatively impervious to influence of aging or an environment, and filtering characteristics thereof can be relatively easily altered. Among various types of digital filters, a finite impulse response (FIR) filter has been preferably employed in view of its excellent stability.
As described above, a natural frequency of a weight sensor is subject to an article weight and a tare. Thus, if an article weight or a tare is changed, it results in change in a frequency of a natural frequency noise associated with the weight sensor. Formerly, change in frequency of a natural frequency noise caused due to change in article weight was negligible because a tare, e.g., a weight of a conveyor, was dominant. However, in recent days, there has been arisen a demand for accurate measurement of a weight of a light article, which involves reduction of a tare. For this reason, such change in frequency of a natural frequency noise caused due to change in article weight has become too significant to neglect. On the other hand, a frequency of a vibration noise associated with a motor or a transfer roller is changed in accordance with specifications of a transfer system such as a transfer speed, for example. Hence, it is desired to achieve a digital filter which is capable of surely eliminating a vibration noise in spite of change in frequency of the vibration noise which occurs due to change in type or size of an article under measurement, or change in specifications of a transfer system.
In accordance with the conventional practices, a digital filter formed of multistage moving average filters is employed in order to surely eliminate a vibration noise contained in a weighing signal, as taught in Japanese Patent No. 6-21814 (hereinafter referred to as “JP 6-21814”).
More specifically, JP 6-21814 teaches a method in which respective moving average values of the multistage moving average filters are differentiated from one another to provide a plurality of notches in a frequency range within which a frequency of a vibration noise would be changed, thereby surely eliminating the vibration noise.
Additionally, an example of a method of designing an FIR filter is described in “Design of FIR Transmitter and Receiver Filters for Data Communication System” by Naoyuki Aikawa et al. in Transaction of IEICE, Vol. J79-A, No. 3, Mar. 1996, pages 608-615, which shows a technique using successive projections method.
According to the method shown in JP 6-21814, a frequency range within which a frequency of a vibration noise would be changed is presumed and the number of stages of the moving average filters is determined in accordance with the frequency range as presumed, in a step of designing a digital filter. As such, in a situation where the frequency of the vibration noise falls outside the presumed frequency range for some reason such as addition of a specification after designing the digital filter, it is required to design a digital filter all over again.
In this regard, to initially design a digital filter which allows for addition of a specification in a weight measurement apparatus requires preparing numerous multistage moving average filters, which results in increase of a circuit scale of the digital filter.
On the other hand, as an alternative to the method taught in JP 6-21814, there has been proposed a method in which a plurality of filter coefficients representing different attenuation characteristics are previously stored. According to this method, each time a frequency of a vibration noise is changed due to change in specifications or the like, one of the stored filter coefficients is retrieved in accordance with a new frequency of the vibration noise after the change, to be used. However, this alternative requires previously storing numerous filter coefficients in order to surely eliminate a vibration noise at a wide range of frequencies, which results in increase of a circuit scale of a memory for storing the filter coefficients.